Cell Structure: Subcellular Components
AP Biology· AP Biology CED — Cell Structure and Function· 14 min read
1. Structure and Function of Eukaryotic Organelles★★☆☆☆⏱ 4 min
Eukaryotic subcellular components are categorized as non-membrane-bound or membrane-bound. Non-membrane-bound ribosomes are made of ribosomal RNA (rRNA) and protein, assembled into two subunits, and synthesize all cellular proteins, found in all living cells.
Rough Endoplasmic Reticulum (rough ER): Studded with attached ribosomes, modifies and packages newly synthesized proteins for transport.
Smooth Endoplasmic Reticulum (smooth ER): No attached ribosomes; functions in lipid synthesis, detoxification, and calcium ion storage.
Golgi Complex: Flattened membrane sacs (cisternae) with a cis face that receives vesicles and trans face that ships modified products to their final destination.
Mitochondria: Double-membrane organelle with inner membrane folded into cristae; carries out aerobic cellular respiration to produce ATP.
Lysosomes: Membrane-bound sacs of hydrolytic enzymes; digests macromolecules, recycles damaged organelles, and mediates apoptosis.
Chloroplasts: Double-membrane organelle with internal thylakoids stacked into grana; carries out photosynthesis in plants and algae.
Central Vacuole: Large membrane-bound plant cell sac; stores water, ions, nutrients, and maintains turgor pressure.
Pancreatic acinar cells secrete large amounts of digestive enzymes (which are proteins) into the small intestine. Which two organelles would you expect to be far more abundant in pancreatic acinar cells than in a cell that produces only cytoplasmic proteins? Justify your selection.
- 1
First, identify the core requirements of the pancreatic acinar cell: it synthesizes large amounts of secretory proteins for export out of the cell.
- 2
Recall that secretory proteins are synthesized by ribosomes attached to the rough ER, which processes the newly made protein. So rough ER must be highly abundant to support high volumes of secretory protein synthesis and processing.
- 3
After processing in the rough ER, secretory proteins are transported to the Golgi complex, which sorts, modifies, and packages them into secretory vesicles for export. So the Golgi complex must also be highly abundant.
- 4
Other organelles (like smooth ER or mitochondria) support general cell function but are not specifically overrepresented for protein secretion, so the two key organelles are rough ER and Golgi complex.
Exam tip:
Always link your prediction of organelle abundance directly to the cell’s specific primary function. AP exam graders require a clear functional connection, not just a correct organelle name, to award points.
2. Prokaryotic vs Eukaryotic Subcellular Components★★☆☆☆⏱ 3 min
The most fundamental difference between prokaryotic and eukaryotic cells is the presence of membrane-bound organelles in eukaryotes, and their absence in prokaryotes. Prokaryotes (bacteria and archaea) store their circular chromosomal DNA in an unenclosed nucleoid region, rather than a membrane-bound nucleus, and only have non-membrane-bound organelles, primarily 70S ribosomes. Eukaryotes have a membrane-bound nucleus storing linear chromosomes, plus full specialized membrane-bound organelles, with 80S ribosomes in the cytoplasm.
A key exception to this pattern is that mitochondria and chloroplasts in eukaryotic cells retain prokaryotic-like features: 70S ribosomes and circular DNA. This is core evidence for the endosymbiotic theory.
A researcher isolates a pure fraction of 70S ribosomes from a cell lysate and claims the original cell must be prokaryotic. Is this claim necessarily true? Justify your answer.
- 1
Recall the general rule: prokaryotic ribosomes are 70S, while eukaryotic cytoplasmic ribosomes are 80S.
- 2
Recall the exception: eukaryotic mitochondria and chloroplasts are descended from prokaryotes, so they have their own 70S ribosomes for internal protein synthesis.
- 3
The 70S ribosome fraction could easily have been isolated from the mitochondria or chloroplasts of a eukaryotic cell, rather than from a whole prokaryotic cell.
- 4
Therefore, the claim that the original cell must be prokaryotic is not necessarily true.
Exam tip:
AP multiple-choice questions frequently test the 70S ribosome exception. Automatic assumption that 70S = prokaryote will lead to lost points.
3. Endosymbiotic Theory for Organelle Origin★★★☆☆⏱ 4 min
Endosymbiotic Theory
The widely accepted model that mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by a larger ancestral eukaryotic cell, forming a permanent mutually dependent endosymbiosis.
Both mitochondria and chloroplasts have a double membrane: outer from host engulfment, inner from the original prokaryote membrane.
Both organelles have their own circular DNA, matching prokaryotic chromosome structure.
Both have 70S ribosomes, identical to prokaryotic ribosomes.
Both replicate independently of the host cell via binary fission, the same method prokaryotes use.
A student claims that the nucleus of eukaryotic cells originated via endosymbiosis. Evaluate this claim using evidence from subcellular structure.
- 1
First, recall the four key lines of evidence that support endosymbiosis for mitochondria and chloroplasts.
- 2
Compare the nucleus’s structure to these requirements: while the nucleus has a double membrane (the nuclear envelope), it does not have independent 70S ribosomes or circular prokaryotic-like DNA. The nucleus also cannot replicate independently of the endomembrane system.
- 3
Current evidence for nuclear origin points to infolding of the ancestral eukaryote's plasma membrane, not engulfment of a prokaryote.
- 4
Therefore, there is no supporting evidence from subcellular structure to support the claim, so the claim is unsupported.
Exam tip:
On FRQ questions asking for evidence for endosymbiosis, always name at least two specific subcellular features to earn full credit. Vague claims like 'it has prokaryotic characteristics' will not earn points.
4. AP-Style Practice Worked Examples★★★☆☆⏱ 3 min
A researcher observes an unknown cell under an electron microscope and records the following features: 70S ribosomes, a cell wall, circular DNA, and no membrane-bound organelles. Which of the following best classifies this cell?
A) A eukaryotic animal cell
B) A prokaryotic bacterial cell
C) A eukaryotic plant leaf cell
D) A eukaryotic photosynthetic algal cell
- 1
First, eliminate all eukaryotic cell options because the cell explicitly has no membrane-bound organelles, a defining trait of prokaryotes. All eukaryotic cells (options A, C, D) have a membrane-bound nucleus and other membrane-bound organelles, so these are incorrect.
- 2
All observed features match the standard subcellular composition of a prokaryotic bacterial cell. The correct answer is B.
Different human cell types have very different functions, leading to predictable differences in the abundance of specific subcellular components.
(a) Identify one organelle that you would expect to be more abundant in heart muscle cells than in most other human cell types. Justify your answer.
(b) Identify one organelle that you would expect to be more abundant in liver cells than in most other human cell types. Justify your answer.
(c) Identify two organelles that you would expect to be more abundant in ovarian cells that produce the steroid hormone estrogen than in most other cell types. Justify your answer.
- 1
(a) Mitochondria. Heart muscle contracts almost continuously to pump blood, requiring a very large supply of ATP. Mitochondria produce ATP via aerobic cellular respiration, so higher ATP demand requires more mitochondria.
- 2
(b) Smooth endoplasmic reticulum. A core function of the liver is detoxifying harmful compounds that enter the bloodstream, and smooth ER is responsible for detoxification of organic compounds. This high demand for detoxification requires more smooth ER than other cell types.
- 3
(c) Smooth endoplasmic reticulum and Golgi complex. Estrogen is a steroid lipid, which is synthesized by the smooth ER. After synthesis, estrogen must be packaged into secretory vesicles for secretion out of the ovarian cell by the Golgi complex. High levels of steroid hormone production and secretion require elevated abundance of both organelles.
The antibiotic chloramphenicol inhibits protein synthesis by binding specifically to 70S ribosomes, and does not affect 80S ribosomes. A patient is prescribed chloramphenicol to treat a bacterial (prokaryotic) infection, and asks if the drug will kill their own human cells. Evaluate the patient’s concern using your knowledge of subcellular component structure.
- 1
Bacterial cells are prokaryotic, so all of their ribosomes are 70S, meaning chloramphenicol will block their protein synthesis and kill the bacteria.
- 2
Human cells are eukaryotic, so all cytoplasmic ribosomes are 80S, which are not affected by chloramphenicol. While human mitochondria do have 70S ribosomes, therapeutic doses of chloramphenicol only affect the much larger population of bacterial 70S ribosomes, with minimal impact on human mitochondrial function.
- 3
Therefore, the patient’s concern that chloramphenicol will kill their own cells is largely unfounded. Chloramphenicol is selectively toxic to bacteria due to the difference in ribosome size between prokaryotes and eukaryotic cytoplasm.
5. Common Pitfalls
Wrong move:
Claims that ribosomes are membrane-bound organelles found only in eukaryotes
Why:
Confuses endomembrane system organelles with non-membrane-bound organelles; forgets prokaryotes need to synthesize proteins
Correct move:
Always categorize ribosomes as non-membrane-bound and universal to all prokaryotic and eukaryotic cells.
Wrong move:
Claims that all eukaryotic cells contain both mitochondria and chloroplasts
Why:
Overgeneralizes from plant cells to all eukaryotes, forgets animal/fungal cells do not photosynthesize
Correct move:
Remember that all aerobic eukaryotic cells have mitochondria, but only photosynthetic eukaryotes have chloroplasts.
Wrong move:
Assigns core protein synthesis function to the rough ER
Why:
Confuses the role of attached ribosomes with the rough ER’s own function
Correct move:
Always state that ribosomes (not rough ER) carry out protein synthesis; rough ER modifies and packages proteins made by attached ribosomes.
Wrong move:
Justifies a claim with 'prokaryotes have no organelles at all'
Why:
Overgeneralizes the fact that prokaryotes have no membrane-bound organelles
Correct move:
Always specify that prokaryotes lack membrane-bound organelles, but do have non-membrane-bound organelles like ribosomes.
Wrong move:
Provides only one piece of evidence for endosymbiotic theory when the question asks for supporting evidence
Why:
Assumes one point is enough for full credit; forgets AP FRQs require multiple specific supporting points
Correct move:
Memorize three specific evidence points (double membrane, circular DNA, 70S ribosomes) to draw from for any endosymbiosis question.
Wrong move:
Claims the smooth ER synthesizes proteins and the rough ER synthesizes lipids
Why:
Mixes up the core functions of the two ER types
Correct move:
Use the mnemonic 'Rough for Proteins, Smooth for Lipids' to avoid this swap.
6. Quick Reference Cheatsheet
Category | Key Rule | Notes |
|---|---|---|
Ribosomes | Made of rRNA + protein; core function = protein synthesis | Non-membrane bound; 70S in prokaryotes, mitochondria, chloroplasts; 80S in eukaryotic cytoplasm |
Rough Endoplasmic Reticulum | Has attached ribosomes; function = process/transport secretory proteins | Part of the endomembrane system |
Smooth Endoplasmic Reticulum | No attached ribosomes; function = lipid synthesis, detoxification, Ca²+ storage | Abundant in liver and steroid-producing cells |
Golgi Complex | Flattened cisternae; cis = receive, trans = ship; function = modify/sort/package proteins | Required for secretion of products out of the cell |
Mitochondria | Double membrane, inner cristae; function = aerobic cellular respiration, ATP production | Found in all aerobic eukaryotic cells |
Chloroplasts | Double membrane, internal thylakoids/grana; function = photosynthesis | Only found in photosynthetic eukaryotes |
Lysosomes | Membrane-bound, contains hydrolytic enzymes; function = digestion/recycling | Not found in most plant cells |
Prokaryotic Subcellular Components | No membrane-bound organelles; circular DNA in nucleoid | All ribosomes are 70S |
Eukaryotic Subcellular Components | Membrane-bound nucleus and organelles; linear DNA | Cytoplasmic ribosomes are 80S |
Endosymbiotic Theory | Mitochondria/chloroplasts evolved from engulfed free-living prokaryotes | Evidence: double membrane, circular DNA, 70S ribosomes, independent binary fission |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 · MCQ
70S ribosome identification question
- 2022 · FRQ
Endosymbiosis evidence FRQ
Going deeper
What's Next
Mastering subcellular component structure and function is the non-negotiable foundation for the rest of AP Biology Unit 2: Cell Structure and Function. Next, you will explore cell compartmentalization, which explains how membrane-bound organelles increase metabolic efficiency by separating incompatible chemical reactions, and how the unique structure of each subcellular component supports this compartmentalization. Without a solid understanding of each organelle’s structure and role, you will not be able to explain the evolutionary benefits of compartmentalization or connect organelle dysfunction to cellular disease, both common high-weight FRQ topics. Later in the course, this knowledge is also critical for understanding cellular respiration and photosynthesis, two major units that make up a large portion of your exam score.